The present invention is related to a vibrating type angular velocity sensor utilizing characteristics of a gyroscope.
Currently, great attention is paid to sensors capable of detecting angular velocities of moving objects, in particular, sensors manufactured by using semiconductor micromachining techniques. These sensors own merits, e.g., compact devices, mass productivities, high precision, and high reliability.
FIGS. 6A and 6B schematically represent a typical vibrating type gyroscope manufactured with employment of the micromachining processing for the semiconductor device as described in, for instance, the publication entitled "MICROMECHANICAL TUNING FORK GYROSCOPE TEST RESULTS" written by M. Weiberg et al., AIAA-94-3687-CP. FIG. 6A is a plan view, and FIG. 6B is a cross-sectional view, taken along a line B--B shown in FIG. 6A. Two vibrator elements 2 are supported via an anchor portion 3 by a beam 4 on a substrate 1. A displacement detection electrode 7 is formed on a glass or silicon on the substrate 1 under these vibrator elements 2. Both the vibrator elements 2 and the displacement detection electrode 7 will constitute a capacitor C1 and a capacitor C2 as represented in FIG. 7.
The right/left vibrator elements are excited along an X-axial (inner plane direction) direction by applying a DC voltage and an AC voltage to a driving comb electrode 5. A so-called "tuning fork drive" is employed so as to realize reversal phases whose phases are different from each other by 180 degrees at a resonant point in a vibration system. A drive-displacement detecting fixed electrode 6 is positioned opposite to the respective vibrator elements 2 in order to monitor vibrations of these vibrator elements 2 to be driven.
On the other hand, when an angular velocity ".OMEGA." around a Y axis is exerted on the right/left vibrator elements 2 which are excited in the reversal phases along .+-.X directions (substrate inner plane direction), each of these vibrator elements 2 receives inertial force (namely, Coriolis force) along a Z-axial (substrate outer plane) direction, and this inertial force is directly proportional to mass "m", a velocity "v", and an angular velocity ".OMEGA." of the vibrator element 2. In response to this inertial force, twist vibrations are induced around the Z axis as a center. In response to Z-directional displacement of the vibrator elements caused by the twist vibrations, a capacitance value between one vibrator element 2 and the displacement detection electrode 7 is increased (C1+.DELTA.C1), whereas a capacitance value between the other vibrator element 2 and the displacement detection electrode 7 is decreased (C2-.DELTA.C2). A change in the capacitance values is converted into a voltage by a C-V converter, and this voltage is sync-detected by using the effect frequency of the Coriolis force, so that it is possible to produce such a sensor output proportional to the angular velocity ".OMEGA.".
In the above-described conventional system, in order that the vibration amplitudes of the vibrator elements 2 are increased and furthermore the displacement sensitivities along the detections are improved, large characteristic values "Q" of vibrations are required even in the vibrations along any directions. For example, the above-mentioned prior art gyroscope reports that the characteristic value Q in the excitation direction is 40,000 and the characteristic value Q in the detection direction is 5,000 under pressure of 100 mTorr. In particular, along the detection displacement direction, since the opposite area between the vibrator elements 2 and the lower electrode 7 is large and the gap (space) between them is small (on the order of several micrometers), the characteristic value Q becomes small along the drive direction due to so-called "squeeze damping".
On the other hand, since the characteristic value Q is lowered under atmospheric pressure, such high voltages as DC 30 V and AC 30 V are required in order to obtain the drive displacement having the similar degree.
As described above, in the prior art system, the sensor element is necessarily required by way of the vacuum sealing (encapsulation) so as to lower the drive voltages and increase the detection displacement sensitivities. As a result, air-tight vacuum packaging is required. To secure the reliability thereof, there is such a drawback that the metal packages are required, namely higher cost.
Even when the sensor is used under atmospheric pressure, the high drive voltages are required. Since this sensor owns such a structure that the Z-axial direction is employed as the detection vibration direction, as previously explained, there is another problem that the characteristic value Q along the detection direction is small, as compared with the characteristic value Q along the excitation direction. A further problem is such that sufficiently high detection sensitivities could not be achieved by only increasing the drive voltages.
Furthermore, generally speaking, in a vibrating type gyroscope, a resonant point of drive vibrations must be slightly shifted from a resonant point of detection vibrations in order to maintain a frequency characteristic of this vibrating type gyroscope. A resonant frequency of displacement vibrations along the Z-axial direction depends upon a thickness of this structure, so that precise thickness controls should be required. In the case that a sensor element is manufactured by using the lithography in the semiconductor field, although better patterning precision within the plane can be achieved, the control along the thickness direction (etching control and the like) becomes difficult. There is another problem that it is practically difficult to properly set the resonant frequencies of both members.